1. Field of the Invention
This invention relates to a new and useful improvement in preparing a crystalline [metallo]aluminophosphate composition. The improvement resides in a calcination method which avoids or minimizes structural changes during calcination by drying the composition, followed by heating the composition to a calcination temperature at a rate no greater than 10.degree. C./minute with a high flow rate of a non-oxidizing gas, e.g. nitrogen, and thereafter with an oxygen-containing gas, e.g. air, at high gas flow rates, e.g., 100 to 400 cc/minute per gram.
This invention is particularly suited to treating as-synthesized crystalline [metallo]aluminophosphate composition comprising crystals having large pore windows measuring greater than about 10 Angstroms in diameter, such as, for example, greater than about 12 Angstroms in diameter. The resulting product is particularly suited to use in hydrocarbon conversion processes.
2. Discussion of the Prior Art
Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversion. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure as determined by X-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties.
Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline aluminosilicates. These aluminosilicates can be described as rigid three-dimensional frameworks of SiO.sub.4 and AlO.sub.4 in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total aluminum and silicon atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, for example an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of aluminum to the number of various cations, such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation may be exchanged either entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given aluminosilicate by suitable selection of the cation. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration.
Prior art techniques have resulted in the formation of a great variety of synthetic zeolites. The zeolites have come to be designated by letter or other convenient symbols, as illustrated by zeolite A (U.S. Pat. No. 2,882,243), zeolite X (U.S. Pat. No. 2,882,244), zeolite Y (U.S. Pat. No. 3,130,007), zeolite ZK-5 (U.S. Pat. No. 3,247,195), zeolite ZK-4 (U.S. Pat. No. 3,314,752), zeolite ZSM-5 (U.S. Pat. No. 3,702,886), zeolite ZSM-11 (U.S. Pat. No. 3,709,979), zeolite ZSM-12 (U.S. Pat. No. 3,832,449), zeolite ZSM-20 (U.S. Pat. No. 3,972,983), zeolite ZSM-35 (U.S. Pat. No. 4,016,245), zeolite ZSM-38 (U.S. Pat. No. 4,046,859), and zeolite ZSM-23 (U.S. Pat. No. 4,076,842) merely to name a few.
Aluminum phosphates (which are included in the term [metallo]aluminophosphates by virtue of the bracketing of "metallo" which indicates its presence as optional) are taught in U.S. Pat. Nos. 4,310,440 and 4,385,994, for example. These aluminum phosphate materials have essentially electroneutral lattices. U.S. Pat. No. 3,801,704 teaches an aluminum phosphate treated in a certain way to impart acidity. U.S. Pat. No. 4,673,559 teaches numerous different silicoaluminophosphate structures. Examples 17 and 18 of that patent show synthesis of MCM-9, a silicoaluminophosphate composition containing crystals having large pore windows measuring between 12 and 13 Angstroms in diameter.
U.S. Pat. No. 4,880,611 teaches compositions which comprise crystals having large pore windows of at least about 10 Angstroms, e.g. from about 12 to about 13 Angstroms, diameter. An example of such a composition is a large pore crystalline [metallo]aluminophosphate which has a framework topology after heating at 110.degree. C. or higher giving and X-ray diffraction pattern with interplanar d-spacings at 16.4.+-.0.2 Angstroms, 8.2.+-.0.1 Angstroms, 6.21.+-.0.05 Angstroms, 6.17.+-.0.05 Angstroms, 5.48.+-.0.05 Angstroms and 4.74.+-.0.05 Angstroms, and without a significant interplanar d-spacing at 13.6-13.3 Angstroms. This material and its method of preparation are further set out in the aforementioned U.S. Pat. No. 4,880,611 which is incorporated herein by reference. Such a composition can be prepared from a reaction mixture hydrogel containing sources of aluminum oxide, phosphorus oxide, water, and a directing agent DA, preferably tetrapropylammonium.
An early reference to a hydrated aluminum phosphate which is crystalline until heated at about 110.degree. C., at which point it becomes amorphous, is the "H.sub.1 " phase or hydrate of aluminum phosphate of F. d'Yvoire, Memoir Presented to the Chemical Society, No. 392, "Study of Aluminum Phosphate and Trivalent Iron", Jul. 6, 1961 (received), pp. 1762-1776. This material, when crystalline, is identified by the Joint Commission for Powder Diffraction Standards (JCPDS), card number 15-274, and has an X-ray diffraction pattern exhibiting lines of Tables I and II, hereinafter presented. Once heated at about 110.degree. C., however, the d'Yvoire material becomes amorphous. The 18-membered ring aluminophosphate VPI-5 was published by M. Davis et al. at the "Innovation in Zeolite Materials Science" meeting in Sep., 1987. A further description of this material and its metal substituted aluminumphosphate counterparts can be found in published PCT application WO 89/ 01912, International Application Number PCT/US 88/02910 to Davis et al, filed Aug. 24, 1988 and published Mar. 9, 1989. U.S. Pat. No. 4,673,559 discloses the 18-membered ring silicoaluminophosphate MCM-9.
Silicoaluminophosphates of various structures are taught in U.S. Pat. No. 4,440,871. Aluminosilicates containing phosphorous, i.e. silicoaluminophosphates of particular structures are taught in U.S. Pat. Nos. 3,355,246 (i.e. ZK-21) and 3,791,964 (i.e. ZK-22). Other teachings of silicoaluminophosphates and their synthesis include U.S. Pat. No. 4,673,559 (two-phase synthesis method); 4,623,527 (MCM-10); 4,639,358 (MCM-1); 4,647,442 (MCM-2); 4,664,897 (MCM-4); 4,639,357 (MCM-5) and 4,632,811 (MCM-3).
A method for synthesizing crystalline metalloaluminophosphates is shown in U.S. Pat. No. 4,713,227, and an antimonophosphoaluminate and the method for its synthesis are taught in U.S. Pat. No. 4,619,818. U.S. Pat. No. 4,567,029 teaches metalloaluminophosphates, and titaniumaluminophosphate and the method for its synthesis are taught in U.S. Pat. No. 4,500,651.
The phosphorus-substituted zeolites of Canadian Patents 911,416; 911,417 and 911,418 are referred to as "aluminosilicophosphate" zeolites. Some of the phosphorus therein appears to be occluded, not structural.
U.S. Pat. No. 4,363,748 describes a combination of silica and aluminum-calcium-cerium phosphate as a low acid activity catalyst for oxidative dehydrogenation. Great Britain Patent 2,068,253 discloses a combination of silica and aluminum-calcium-tungsten phosphate as a low acid activity catalyst for oxidative dehydrogenation. U.S. Pat. No. 4,228,036 teaches an alumina-aluminum phosphate-silica matrix as an amorphous body to be mixed with zeolite for use as cracking catalyst. U.S. Pat. No. 3,213,035 teaches improving hardness of aluminosilicate catalysts by treatment with phosphoric acid. The catalysts are amorphous.
U.S. Pat. No. 2,876,266 describes an active silicophosphoric acid or salt phase of an amorphous material prepared by absorption of phosphoric acid by premolded silicates or aluminosilicates.
Other patents teaching aluminum phosphates include U.S. Pat. Nos. 4,365,095; 4,361,705; 4,222,896; 4,210,560; 4,179,358; 4,158,621; 4,071,471; 4,014,945; 3,904,550 and 3,697,550.
The [metallo]aluminophosphates are generally prepared from a reaction mixture which contains an organic directing agent which requires removal in order to provide a composition suited for catalysis. Commonly, such removal is effected by calcining. However, some [metallo]aluminophosphates are susceptible to structure alteration upon calcination by standard techniques. Accordingly, a calcination technique which does not cause substantial structural alteration would be particularly desirable for preparing [metallo]aluminophosphates for use in hydrocarbon conversion processes.